   ([^String s ^long width]
    (center s width " "))
   ([^String s ^long width ^String padding]
-   {:pre [(not-empty padding)]
+   {:pre [(not-empty padding)
+          (not (nil? s))]
     :post [(= (.length %) width)]}
    (if (<= width (.length s))
      s

  ([^String s ^long width]
   (pad-left s width " "))
  ([^String s ^long width ^String padding]
   {:pre [(not-empty padding)]
    :post [(= (.length %) width)]}
   (if (<= width (.length s))
     s

  ([^String s ^long width]
   (pad-right s width " "))
  ([^String s ^long width ^String padding]
   {:pre [(not-empty padding)
          (not (nil? s))]
    :post [(= (.length %) width)]}
   (if (<= width (.length s))
     s
